1. Field of the Invention
The present invention relates to a power supply circuit of an audio signal amplifier or an amplifier having a similar operation, such as a motor drive amplifier, and the like.
2. Description of Related Art
Conventionally, there have been proposed various power supply circuits of an audio amplifier or a motor drive power amplifier and so on, aiming at the reduction of electric power consumption, or the reduction of the size of the power supply circuit as well as its weight.
In the example of a power supply circuit of an audio amplifier, when a high level output signal is produced, the amplitude of the output signal is limited by the voltage of the power-supply being used, although the power-supply voltage is sufficient when the level of the output signal is low. This means that the magnitude of the maximum output signal obtainable from a power amplifier is determined by the power-supply voltage. If a power supply of a high voltage is used so that a sufficient output amplitude is obtained by the amplifier when a high level output signal is produced, the electric power consumption of the power-supply circuit and the power amplifier under the normal operating condition becomes large, resulting in a decrease in efficiency.
In order to eliminate this shortcoming, various systems have been proposed as illustrated in FIG. 1A though FIG. 1C.
The example shown in FIG. 1A is a system which is configured that the power supply voltage is controlled so that it follows the envelope of the peak values of the signal when the output signal is higher than a previously specified value. The example shown in FIG. 1B, on the other hand, is a system configured that the power-supply voltage is controlled to follow the amplitude of the amplifier's output signal.
The example shown in FIG. 1C is a system configured that a plurality of power-supply voltages are supplied for the output signal higher than the specified output signal.
FIG. 2 shows an example of a power-supply circuit for concretely showing the cases shown in FIGS. 1A through 1C.
Referring to FIG. 2, the operation of a power-supply circuit of a voltage control type will be generally explained.
As shown in FIG. 2, a source terminal of a p-type MOSFET (hereinafter, abbreviated as PFET) is connected to a positive terminal of a battery 1 (Vb) which is provided as a power-source. A resistor 3 and a PWM oscillator 4 are connected across the gate and source of the PFET 2, so as to constitute a switch device in which the PFET 2 is switched ON and OFF by the signal from the PWM oscillator 4.
The drain terminal of the PFET 2 is connected to a terminal of a coil 5 whose the other terminal is connected to a ground terminal, and also to a cathode terminal of a diode 6. A capacitor 7 is connected across the anode terminal of the diode 6 and the ground terminal.
To the battery 1, a terminal of a coil 8 is connected, whose the other terminal is connected to a drain terminal of an n-type MOSFET (hereinafter, abbreviated as NFET) 9 and an anode terminal of a diode 10. A resistor 11 and an PWM oscillator 12 are connected across the gate and source of the NFET 9, that is, across the gate of NFET 9 and the ground, so that a switch element in which the NFET 9 is switched ON and OFF by a signal from the PWM oscillator 12 is provided.
A capacitor 13 is connected across the cathode and anode of the diode 10. A capacitor 14 and a load (RL) 15 are connected across the cathode of the diode 10 and the anode of the diode 6.
The power-supply circuit shown in FIG. 2 is a plus-minus two powers supply circuit which is constituted by a minus-chopper circuit generating a minus voltage by the PFET 2, coil 5, diode 6 and capacitor 7, and a plus-chopper circuit generating a plus voltage by the NFET 9, coil 8, diode 10 and capacitor 13.
In the plus-chopper circuit, the current supplied from the battery 1 is accumulated in the coil 8 when the NFET 9 driven by the PWM oscillator 12, which function as a switch element, is turned ON. When the NFET 9 is switched OFF subsequently, a voltage produced by a counter electromotive force of the coil 8 is superimposed on the voltage of the battery 1, so that a positive voltage whose level is higher than the battery voltage (Vb) is produced at a terminal "c" shown in the drawing.
The magnitude of the counter electromotive force produced in this instance is controlled by the pulse width of the pulse signal supplied from the PWM oscillator 12. More particularly, if the pulse width is widened to prolong the period in which the switch element is turned ON, a large counter electromotive force will be generated. Conversely, if the period in which the switch element is turned ON is shortened, a small counter electromotive force will be generated. In the minus chopper circuit, the current supplied from the battery 1 is accumulated in the coil 5 when the PFET 2 driven by the PWM oscillator 4, which functions as a switch element, is turned ON. When the PFET 2 is turned OFF subsequently, a counter electromotive force of the coil 5 is generated with respect to ground, so that a negative voltage is produced at a terminal "d" shown in the drawing. Unlike the operation of the plus chopper circuit, the counter electromotive force is produced with respect to ground so that the battery voltage is not superimposed.
As described above, the voltages produced by two types of chopper circuits are controlled by the pulse widths of the PWM oscillators provided for the switch elements respectively. Therefore, the power-supply voltage can be varied in response to the output signal as depicted in FIGS. 1A through 1C by detecting the output signal or an input signal of the power amplifier and controlling the output pulses of the PWM oscillators by means of a control circuit which is not shown in the drawing.
For example, if the input signal of the power amplifier is within a specified signal voltage range, the operation of the PWM oscillator is stopped, and +12 volts, for example, is produced at the terminal "c" and 0V is produced at the terminal "d", so that the power-supply voltage of 12 volts is supplied to the load.
When, on the other hand, the input signal level is above the specified signal voltage range, the pulse signal is produced by a control signal from the control circuit, and supplied to each switch element. As a result, +16 volts, for example, is produced at the terminal "c" and -4 volts is produced at the terminal "d", so that a power supply voltage of 20 volts is supplied to the load.